The present invention relates to a vibration-suppressing sound-shielding board that is available as a building material.
In order to shield a sound generated from the interior of a building so as not to leak out to the exterior, or in order to shield a noise in the exterior of a building so as not to invade into the building, heretofore, a gypsum plaster board, an asbestos cement board, a calcium silicate board, various kinds of plywoods, a metal plate, etc. have been used as a sound-shielding wall material. However, these known board materials are poor in a vibration damping performance, and their loss factors (.eta.) at a frequency in the proximity of 500 Hz, for example, are 0.01 or less. In the proximity of the characteristic frequency of such board material whose vibration-damping performance is poor, a sound-shielding performance is extremely lowered due to resonance phenomena and coincidence phenomena (the phenomena that a sound propagating velocity in the board material coincides with the sound velocity in the air). Accordingly, a problem was present that even if two rooms in a building should be partitioned by means of a sound-shielding wall, a sound of a piano played in one room would be heard in the other room through the sound-shielding wall.
Here, it is to be noted that the loss factor (.eta.) is employed as a measure for hardness of resonance of a vibration system, and a reciprocal of that factor is a Q-value. This Q-value is used as a quantity for indicating a sharpness of resonance when a vibration system resonates, and if a frequency width between two points having an amplitude equal to 1.sqroot.2 times the maximum amplitude value (i.e. an energy equal to 1/2 times the maximum energy) on a resonance curve depicted in the case of varying a frequency in the proximity of a resonance frequency is represented by .DELTA.f (called half-value width), then the Q-value is derived from the equation Q=fo/.DELTA.f, where fo is a characteristic frequency of the vibration system. Accordingly, the larger the loss factor (.eta.) is, the harder is the vibration system to resonate in the case where an external vibrating force is applied to the vibration system.
In order to resolve the above-mentioned problem, it was proposed in the prior art to use a composite board consisting of a plurality of board materials joined with an adhesive or a composite board consisting of a plurality of board materials with a viscoelastic substance (for instance, elastomer) intervening therebetween, as a sound-shielding board (See Japanese Utility Model Publication No. 54-4491). However, the loss factor (.eta.) of the above-described former composite board which simply utilizes an adhesive is not so different from that of a single board, and hence improvements in a sound-shielding performance are very little. On the other hand, the above-referred latter composite board making use of a viscoelastic substance has an excellent vibration-suppressing sound-shielding performance, but nevertheless the viscoelastic substance as disclosed in Japanese Utility Model Publication No. 54-4491 is very complexed in a manufacturing process and expensive in a manufacturing cost, and moreover, since an adhesive must be used for joining the board materials and the viscoelastic substance, a lot of labor is necessitated for manufacturing the composite board. The present invention has been worked out under the above-mentioned background of the art.